Engine control units up to now have been using a plurality of sensor values that are supplied by sensors to the internal combustion engine in order to control or regulate the injection system and the air system. This applies to both Otto and Diesel engines. One possible alternative of carrying out the engine control is to build up the engine control based on a direct combustion chamber signal that is individual to each cylinder, instead of the above-named sensor values, which supply only indirect data on the combustion process proceeding in the cylinders. For this purpose, it is proposed that one use cylinder pressure sensors to measure the cylinder pressure, since these have the greatest measuring accuracy. In the measurement of cylinder pressure, the basic reference variable, the average indicated pressure, represents a measure for the mechanical work performed by the engine. Since, in particular with Otto engines in SI operation (ignition operation), the average indicated pressure fluctuates very greatly from cycle to cycle, regulation based on this therefore has to be designed to be relatively slow. For this reason, a second control system is integrated based on the combustion position for rapid efficiency adaptation.
In this connection, the angle of the maximum cylinder pressure φ(p_max) brought about by the combustion process has turned out to be a further advantageous reference variable, or the angle of the maximum cylinder pressure gradient over the combustion position φ(dp_max/dφ). However, both signals are superposed by the compression curve, that is, by the component of the pressure that is caused by the compression and expansion based on the piston movement in the cylinder. In this context, there exists fundamentally the problem that this feature is difficult to extract if the maximum pressure value is no longer caused by the compression in the cylinder, as is the case, for example, in response to very late combustion positions.
In the potential use of a cylinder pressure-based engine control, particular importance should be placed on combustion position regulation both for the Otto engine, for example, for determining efficiency during conventional SI operation, in controlled self-ignition, and in the stabilization of new combustion methods for fuel reduction, as well as for the Diesel engine, as, for example, for stabilizing new combustion methods for emission reduction.
Up to now, the combustion position features have been ascertained from the pressure curve, by calculating the compression curve with the aid of a so-called heating curve calculation, in order to obtain the cylinder pressure brought about only by the combustion as an output variable for the determination of the cylinder pressure features required. The heating curve calculation is based on the converted energy quantity, from which the work expended during the compression may be ascertained, so that one may conclude what the pressure is that is acting in the cylinder because of the compression. However, implementing such a calculation requires a lot of effort.